1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Prior Art
Conventionally, one of the display devices for displaying images, videos and others, liquid crystal display devices (LCD) that make use of liquid crystal have been known. LCDs have been mostly utilized as display devices for computers, cellular phones, television sets and the like. In a liquid crystal display device, a special liquid is sandwiched and sealed between two glass plates, and when an electric field is applied across the liquid, a change in the orientation of liquid crystal molecules occurs so that the light transmittance of the liquid varies to thereby display an image. In this process, since the liquid crystal itself does not emit light, cathode fluorescent lamps (CFLs) and the like are equipped on the rear side of the liquid crystal as a light source, and this light source is used as a backlight.
Here, a CFL is a light source involving three RGB wavelengths. However, if the power (brightness) of CFL is increased, all the colors are uniformly raised in brightness, so it has been impossible to make correction to one particular color alone.
To deal with this problem, recently, configurations using two kinds of light sources as a backlight have been emerging. For example, there is a configuration in which light emitting diodes (LEDs) are used in combination with CFLs as a backlight (which will be called “hybrid backlight” hereinbelow as appropriate) (see Patent document 1: Japanese Patent Application Laid-open 2004-139876, for example). Specifically, in order to enhance red color of CFLs, red LEDs of a longer wavelength are used to improve color reproducibility with CFLs at the same time.
However, the above hybrid backlight configuration entails the following problem. That is, it has been known that the luminous intensity of CFLs at startup is lower than the designated value. Accordingly, if the user has selected a low brightness for backlighting, the CFLs cannot but present an extremely low luminous intensity. In order to keep the white balance constant, it is necessary to inhibit the luminous intensity of the red LEDs. To achieve this, however, it is necessary to make the current (IF) supplied to the LEDs very low in order to suppress influence on the luminous intensity. In this case, if current IF is set to a markedly low value, there occurs the problem that the LEDs will not light correctly because of an insufficiency of the current supplied to the LEDs.
Particularly, when a CFL having temperature-dependent characteristics having a peak brightness at ambient temperatures of about 30 to 40 deg. C., is started up or is being used at a low ambient temperature, it may present as low a brightness as the half of the brightness when the backlight becomes stabilized after a temperature rise by virtue of the parts being heated. Accordingly, the LEDs that are designated and expected to successfully deal with such CFL characteristics need to have a broader light intensity adjustable range than that of the CFL. However, it is only possible to extract sufficient LED illumination characteristics when a forward voltage of about 1.6 to 1.8 V is applied to each LED element, so that there is a certain limit that the light intensity of LEDs can be adjusted, hence resulting in inability of correct illumination. For example, there have occurred the problems that LEDs cannot be totally turned on and that LEDs are turned on but flickering.
Referring now to FIG. 1, a specific description will be given. In FIG. 1, six LEDs are connected in series. To turn on LEDs, the voltage VF to be applied across a single LED is usually 1.6 to 1.8 [V] while the current IF flowing the LED is about 5 to 10 [mA]. Also, a resistance R for adjusting the current through the LEDs is connected in series. Here, the following description is made assuming that a resistance R of 430 [Ω] is used.
In FIG. 1, when a voltage of 14 [V] is applied across the whole circuit, the voltage VF applied across the series of six LEDs becomes equal to 1.6×6=9.6 [V]. Accordingly, the current IF is calculated as (14−9.6)/430≈10 [mA]. In this case, the LEDs will be turned on correctly.
However, if the voltage V is varied to 11 [V] in order to reduce the LED brightness, the current IF is sharply reduced to (11−9.6)/430≈3 [mA], which cannot turn on the LEDs correctly.
In this way, when a backlight with a series of LEDs is used, there occurs the inherent problem that it is impossible to perform lighting control by voltage control only.